This invention pertains to protection of wooden objects in direct contact with soil from pest invasion and is particularly applicable to protection of wooden utility poles, wooden railroad ties and wooden fence posts.
Preserving wood from decay has been recognized as a problem from ancient times. Noah's wooden ark was preserved with pitch (Genesis 6:14). Roman books on architecture had descriptions "of preserving trees after they are cut, what to plaster or anoint them with, of the remedies against their affirmities, and of allotting them their proper place in the building." (See W. C. Hayes, ed., "Extending Wood Pole Life: Solving a $5-billion/year Problem", ELECTRICAL WORLD, 41-47 at 42 (February 1986).
In modern times, the protection of wooden utility poles, railroad ties and fence posts from decay has become a major concern. The decay of such wooden objects has been found to be primarily caused by the action of pests and particularly of fungi, termites, carpenter ants, and other wood invading insects.
The decay caused by fungi is a common and an important source of deterioration of wooden objects. (See R. A. Zabel et al., The Fungal Associates, Detection, and Fumigant Control of Decay in Treated Southern Pine Poles, Final Report EL-2768 for EPRI Research Project 1471-1, State University of New York 1982). Although decay most frequently occurs within 50 centimeters of the ground line, any part of the pole which has a moisture content of above 20% and is in contact with oxygen can harbor decay-producing fungi. The secondary region of decay is the cross-tie inter-section area. The fungi feed on wood by extending networks of minute, thread-like strands of single cells (hyphae) through the cracks in the wood. The hyphae secrete enzymes that dissolve the cellulose and lining in the wood, transforming them into simple chemicals that the fungi then use as food. In its incipient stages, decay is often invisible to the naked eye, but it is capable of completely destroying large volumes of wood. The termites, carpenter ants and other wood invading insects bore into the wood thereby destroying its integrity and structural strength. The problem of invasion by pests is exacerbated by the cracking of wood upon drying. As wood dries to below about 30 percent moisture content, it shrinks. Since the moisture level of freshly-cut wood decreases with the distance form the center, as the wood dries, it produces V-shaped cracks, which expose additional surface for penetration by pests. Additionally, any protection of a wooden object which is limited to the outside surface of such object is rendered inoperative once cracks are formed.
The magnitude of the problem of decay of wood is best illustrated by focusing on wooden utility poles. There are about 120 million wooden utility poles in service in the United States, of which 15 to 20 million are currently in need of treatment to remain in service, and 4 to 6 million more become defective each year. A survey by the Electric Power Research Institute ("EPRI") indicated that, on average, it costs $810 to replace an electric distribution pole, and $1690 to replace an electric transmission pole.
The presently accepted commercial approach to protection of new utility poles involves pressure treatment of the outer layers of the lower portions of poles with various organic or inorganic compounds. A creosote, pentachlorophenol ("penta") arsenicals or fluorides. One widely used preservative is creosote, produced by the destructive distillation of coal. Another organic preservative that has been commonly used to impregnate wooden objects including utility poles is pentachlorophenol ("penta"). However, its use in the United States has been severely restricted by the U.S. Environmental Protection Agency. Wooden poles are also impregnated with inorganic compounds, such as, chromated cooper arsenical (CCA), ammoniacal copper arsenate (ACA) or ammoniacal copper zinc arsenate (ACZA) compounds. A problem with these inorganic wood impregnants, however, is that they leach out and quickly lose their effectiveness in preserving the wood.
A problem common to treatment of wood by impregnation with either organic and inorganic preservatives is that the impregnants reach only the surface layers of the wooden objects. Accordingly, wood cracking exposes untreated areas which are subject to decay.
The pressure impregnation approach provides limited decay protection for a few years up to generally about 15 years. Moreover, the pressure impregnation approach cannot be applied to wooden poles already in place. The decay protection of poles already in place may be extended by periodic inspection and treatment, as necessary, with the fumigants, such as chloropicrin (trichloronitromethane), VAPAM (sodium methyldithiocarbamate) a non-volatile solid which is hydrolyzed to form (methyl isocyanate) or VORLEX a volatile liquid containing the active ingredient of methyl isocyanate in conjunction with physical strengthening of the deteriorated pole. Such remedial treatment has been shown to arrest fungal activity in Douglas fir poles for up to 10 years. (See R. D. Graham et al., Controlling Biological Deterioration of Wood with Volatile Chemicals, EPRI Report EL-1480 (Oregon State University, 1980). The treatment with fumigants generally involves drilling a hole at ground level downward and toward the center of the pole and pouring of the fumigant into the hole. The physical strengthening of the deteriorated pole generally lo involves placing reinforcing structures, such as, metal sheath, concrete poured jackets or an adjacent supporting pole.
The problem with the current treatment and repair methods is that they are effective for relatively short periods of time and necessitate regular costly, manpower intensive inspections and continual further treatments and repairs. Providing an excess quantity of an impregnant or a fumigant does not solve the problem of the short duration of the protection. The excess of such impregnant of fumigant is rapidly lost to the air and soil decreasing the long-term effectiveness. Moreover, losses of impregnants or fumigants may cause significant environmental problems. Also, additional impregnants and fumigants are subject to decomposition, which renders them ineffective in the long run and not cost effective in the short run. The concentration of active ingredients resulting from a single application of an impregnant or fumigant starts out well above the minimum level necessary for effectiveness, but decreases rapidly with passing time, dropping quickly below the minimum effective level.
Since a long-term solution to pesticide intrusion is desired, the pesticide which is used to control such intrusion can be incorporated into a controlled release device. A "controlled release device" refers to a substance that results in controlled and sustained release of an active chemical from its surface. The device provides a method for the controlled release of the chemical into the surrounding environment. The chemical released into the environment establishes an effective zone of action.
Presently, there are three controlled release packaging systems including microcapsules, coated granules and chemically-bound fungicides.
While there are a number of reasons for recommending microencapsulation (it is highly versatile, makes use of a variety of manufacturing techniques, and reduces the toxicity of the contained material), it is essentially a short-term system, with lifetimes measured in months rather than years. Additionally, microencapsulation can add significantly to the cost of the fungicide being encapsulated. Furthermore, this process has no use in protecting the other portion of the pole.
The second system involves coated granules. In this case, the fungicide is absorbed onto a matrix such as clay and then coated with cross-linked resins which helps slow the release rate. The presence of relatively large amounts of clay help to reduce the amount of fungicide that can be contained thereby limiting the lifetime of device.
The third system involves chemically-bound fungicides. The fungicide is chemically bound to a polymer either by being reacted with a performed polymer or the fungicide is attached to the monomer and then reacted to form the polymer. As in the case of the coat granules, the lifetime of the system is limited because of the presence of large amounts of polymer which lowers the amount of fungicide that can be contained.
There is, therefore, a long felt and unsatisfied need for a device, a method and a system of preserving wooden objects in contact with soil for a prolonged period of time, by preventing decay and deterioration of such objects by pests such as fungi, termites, ants and other wood invading objects. The need is particularly keen in connection with the prevention of decay and deterioration of wooden utility poles, railroad ties and fence posts.